This invention relates in general to a single-staged thermoelectric cooler for stabilizing the temperature of an uncooled, infrared detector and, in particular to, a high-strength, composite thermoelectric cooler that resists fracture due to compressive or tensile stresses and dampens shearing forces.
Every object, whether cold or hot, emits electromagnetic radiation. The radiation spectrum, however, for hot objects differs from that of cold objects. For example, the sun emits much of its radiation as visible light (0.4 xcexcm to 0.7 xcexcm wavelength). While colder objects, such as people, trees and automobiles, emit most of their radiation in the lower energy, infrared (IR) part of the spectrum (3 xcexcm to 12 xcexcm wavelengths). Since the human eye cannot detect this low energy radiation, IR-sensitive detectors must be used to visually represent IR radiation.
Conventional infrared detectors, known as photon detectors, produce an electrical response directly as a result of absorbing IR radiation. These detectors are strongly dependent on temperature. It is necessary to cryogenically cool these detectors to temperatures of approximately 80 K (xe2x88x92193xc2x0 C.) in order to maintain high sensitivity.
An alternate type of IR sensor uses a thermal detector. These detectors do not require cryogenic temperatures to operate. Significant advances, in both simplicity and performance, have been achieved in this uncooled infrared technology field over the past several years. Uncooled IR systems have many advantages over conventional cooled IR systems, including cost, weight, size and power consumption. In addition, uncooled IR detection technology has allowed development of IR systems for commercial and military applications where low-cost, light weight, high reliability and low power consumption are critical requirements. These applications include surveillance devices, man-portable weapon sights, driver""s aids, and seekers for missiles and smart submunitions.
However, like their cooled IR systems counterparts, the uncooled IR systems are temperature sensitive. The uncooled IR systems use thermal detectors to absorb IR radiation. The IR radiation causes the thermal detectors to experience a temperature change which in turn creates an electrical response which can be displayed on a video monitor. For proper operation, these detectors must be thermally isolated from their immediate surroundings to maximize the temperature change that results from the absorption from a small amount of IR radiation. In order to stabilize the temperature of the IR detector, current systems employ a thermoelectric cooler along with a temperature sensor. With the use of the thermoelectric cooler, the IR detector can remain at the optimum detector operating temperature for peak performance over varying ambient temperatures. This optimum temperature for uncooled IR systems is approximately room temperature, or 295 K.
Thermoelectric coolers are well-known in the art. Typical thermoelectric coolers use arrays of thermocouples which operate using the Peltier or Seebeck effects. The thermocouples are formed from a P-type thermal element and an N-type thermal element which have long been known for producing heating or cooling. These thermocouples generally use a P-type semiconductor or thermal element connected to an N-type semiconductor or thermal element to form a thermoelectric element. Thus, depending on the direction of the current flowing across the N and P junctions, the device may produce heating or cooling at the junction.
Typical single-staged thermoelectric coolers have two ceramic plates, a cold plate and a hot plate, located on either end of the thermoelectric elements. Depending upon the direction of the current, heat will be pumped from one plate to the other. Typically the top surface, the cold plate, will be held at a constant temperature. A temperature sensor on the cold plate sends signals to a power supply to control the direction of current flow which in turn controls the direction of heat flow between the cold plate and the hot plate.
An important characteristic of thermoelectric coolers are their efficiency ratings, which are inversely related to the thermal conductivity of heat between the cold plate and the hot plate. Thermal conductivity, and therefore efficiency, are related to the size of the thermoelectric elements, the number of elements and the air gap between the two plates. For example, the larger the air gap between the two plates, the lower the thermal conductivity and the higher the efficiency rating. In typical uncooled infrared detector applications, optimum efficiency can be reached when the thermoelectric cooler is placed in a ceramic package and the air is evacuated from the system.
The use of such uncooled IR detection systems, however, is limited due to the brittle nature of the thermoelectric elements which may result in fracture or breakage under rough handling or use in hostile environments. Such failures within the thermoelectric elements are typically caused by small shifts in the ceramic plates resulting in shearing forces within the thermoelectric elements. Also of concern, but less common, is failure due to compressive or tensile stresses within the thermoelectric elements.
In order to increase the strength of the thermoelectric coolers, conventional devices have increased the size of the thermoelectric elements. However, with an increase in size of the thermoelectric elements comes a decrease in thermal efficiency of the thermoelectric cooler. Other devices have potted the cooler with an epoxy resin which greatly increases the strength of the thermoelectric cooler, however, this approach also results in a decrease in efficiency of the thermoelectric cooler. A need has therefore arisen for a thermoelectric cooler for stabilizing the temperature of an uncooled infrared detector having high strength without having a loss in efficiency.
The present invention disclosed herein comprises a high-strength, single-staged, composite thermoelectric cooler for stabilizing the temperature of an uncooled, infrared detector. The composite structure of the thermoelectric cooler resists compressive and tensile stresses and dampening shearing forces. Complementing the high-strength feature of the composite structure, this thermoelectric cooler operates at a high level of efficiency.
In accordance with one aspect of the present invention, the high-strength, composite thermoelectric cooler comprises a pair of parallel ceramic plates, a cold plate and a hot plate. A plurality of thermoelectric elements is thermally coupled between the cold plate and the hot plate. The thermoelectric elements are made of N and P type semiconductor material such as bismuth telluride (Bi2Te3). The unoccupied volume between the cold plate and the hot plate defines a plurality of chambers. These chambers are substantially filled with a thermoelectric insulator creating a composite structure which resists compressive and tensile stresses and dampens shearing forces.
To maintain a suitable efficiency rating, the thermoelectric insulator material has a very low density and a very low thermal conductivity similar to that of air. This low thermal conductivity allows the thermoelectric cooler to operate at an efficiency rating substantially the same as that for a thermoelectric cooler operating with an air gap between the ceramic plates, noting that operating in a vacuum the thermal conductivity of air will increase. The thermoelectric insulator can be selected from a group consisting of an aerogel, a xerogel or other similar porous material having a low thermal conductivity.
In accordance with another aspect of the current invention, a wet precursor gel is potted into the chambers between the ceramic plates in the thermoelectric cooler. At a point above the critical point of the gel, pore fluid is extracted from the gel forming the aerogel within the thermoelectric cooler thereby creating a high strength composite structure. In accordance with another aspect of the present invention, liquid CO2 is used to replace pore fluid in the wet precursor gel. At a point above the critical point of CO2, the CO2 is extracted from the gel forming the aerogel between the two ceramic plates thereby creating a high strength composite structure within the thermoelectric cooler.
In yet another aspect of the present invention the wet precursor gel is placed in a mixture with a surface modifying compound and a solvent. This mixture is washed and potted into the chambers between the ceramic plates in the thermoelectric cooler. The mixture is dried in the chambers under ambient conditions forming an xerogel in a composite structure inside the thermoelectric cooler.